Glycoproteins mucin oligosaccharide structure

Figure 14.2 Representative oligosaccharide structures found on mammalian glycoproteins and glycolipids. The complex oligosaccharides may be bi-, tri-, or tetra-antennary the branches may be more or less elongated with 1—>4 linked lactosamine units, and they may or may not be sialylated. The SLex, Lea, and Leb structures represent the different blood group determinants often present on lipids, and the elongated core 2 structure is a mucin-type glycosylation. Proteoglycans have a common core to which a variety of linear acidic polysaccharides are attached.

The enzymes catalyzing reactions (1) and (2) both add sialic acid to j8-linked galactosyl residues however, they are distinct and separate enzymes. Again, as with the glycoprotein and mucin sialytransferases, the enzyme discerns the fine structure of the acceptor molecule. Further, these sialyltransferases will not transfer sialic acid to )8-linked galactosyl residues of glycoproteins, mucins, or oligosaccharides. The third enzyme catalyzes the addition of sialic acid to another sialic acid residue. Little is known about this enzyme since its activity is very low. It is not clear at this time whether this enzyme catalyzes the addition as shown in reaction (3) or utilizes the product of reaction (1) as its substrate. 

Figure 47-2. Structures of two 0-linked oligosaccharides found in (A) submaxillary mucins and (B) fe-tuin and in the sialoglycoprotein of the membrane of human red blood cells. (Modified and reproduced, with permission, from LennarzWJ The Biochemistry of Glycoproteins and Proteoglycans. Plenum Press, 1980.)...

Ten sialic-acid-containing oligosaccharides and four neutral oligosaccharides have been purified from the major glycoprotein fraction of hog, submaxillary-gland mucin. H-N.m.r. spectroscopy at 360 MHz, in combination with methylation analysis and mass spectrometry, allowed the determination of their structures, which are partial structures of a pentasaccharide (see Fig. 42) having blood-group A activity.221... 

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